Beginner Bike Question

The geometry between the two is different enough that I think you would need to ride them back-to-back to make the call yourself. The big thing is that the Roubaix has a taller "stack" than the Diverge, by 8mm in a size 54. Is that a lot? Maybe, it might just be the deciding factor. You also have a different parts spec between the two you cited: the diverge uses 9-speed Sora, the Roubaix using 10-speed Tiagra. That extra gear might make a difference to you. Worth the difference in price? Only you can answer a "worth it" question. In reality, either one will allow you to make it to the finish line, and either one will likely out perform anything on the market 20 years ago. 

Your going to do just fine with the 'all in one' style bike you have. Don't think of it as a gravel bike at all. Also keep in mind the other costs that are going to go with it. Your going to need pedals and clip in style shoes. If your going to be doing triathlons then you will want to go straight away into a triathlon bike shoe and pedals. Around $150-$200+

As your mileage progresses your going to want to go with a set of aero bars, about another $200+ The aero bars are partly to be aero but honestly its more about taking a load off the arms and upper body. Once you start hitting 40-50 mile rides you will find your arms getting tired from holding your body up, then your just going to be begging to get off the bike, so aero bars.

You are also going to need things like riding gloves and helmet (a quality good fitting helmet). If you plan on riding in cold weather you are going to need shoe covers so you don't freeze your toes numb. Oh ya water bottle cages and water bottles. Tires and inner tubes, oh ya tools and Co2 kit so you can change your own tubes and tires. Oh ya, very important and high quality and brightest flashing tail light you can buy for all those training rides, you need drivers to be able to see you. You don't want some weak dime store tail light that does not get the attention of a distracted driver, expect to spend up to $60-$100.

The nice thing about the gearing you have is climbing hills will be a breeze. You don't need carbon fiber and the extra few ounces saved won't make you any faster if you don't have the fitness and technique anyways. And even then, when you get into the truly competitive ranks, your going to be competing with $8K+ bikes, so save your money and rock what you got.

Now go get fitted and start training, 112 mile bike ride followed by a 26.2 run is going to challenge you like you have never been challenged before. Oh ya, don't forget about that little 2.4 mile swim. 

The bike you picked is just fine. It is only 1 small part of the Triathlon equipment list. You still need to get all your swim gear for training.

Not that it matters, but for comparison I ride a 1988 steel framed road bike that weighs in at 22lbs, been through 9 triathlons in less them 3 years. Paid $150 for it and have $1,500 invested in it. Works just fine for me, hoping to take it to Ironman Boulder next year

Best Wishes!

While it's great to have the aero bars to give you a position option, if your road bike position has you putting so much weight on your arms that you can't handle a 40-50 mile ride after several months of riding, then you've got some issues. That may be lack of specific core strength to hold your upper body (and extensive use of aero bars will do wonders for destroying that specific core strength and muscular coordination). It may be that your position is too far forward at the saddle (if you ride on the hoods, then take your hands off the bars without sitting up, you should immediately feel like you're falling forward). It may be a flexibility issue. But, properly set up, and a properly trained and balanced core, that kind of mileage shouldn't have you begging to get off the bike.

Hello, I certainly appreciate the feedback as it is something I have struggled with in terms of bike fitment. I should remember that my particular fit and reasons for wanting aero bars (which I currently do not have, but are on my soon to purchase list) are not reasons another person would want or need the same equipment.

As to your insights if I might respond to give abit of background. I use my bike for training and racing and have my front bars dropped as low as they go, my position is indeed abit forward but not due to me riding on the hoods. When in a aggressive position I am in the bottom drops and my jaw only inches from the bar. The reason for my more forward position is stability. I used to attack downhill mountain road twisties and gain speeds of up to 50+ mph, on a 30yr old steel framed road bike mind you. If my body position was rearward there was not enough weight on the front of the bike and I would get absolutely terrifying speed shakes, thus I moved my position forward to load the front wheel and that made a major improvement in stability. 

In terms of core fitness and muscle stability I think I am alright. I typically average about 1,000 training miles per year, I currently spend roughly about 3 hours a week in the gym. Now I will admit I do struggle abit with a mid life gut bulge and am about 20lbs overweight. 

Also having a balanced life of full time job and school plus trying to squeeze in training time, well I suppose I am not in the best cycling fitness I could be and in my humble opinion I think it would be great to have a set of aero bars to help extend my mileage by reducing arm fatigue.

I do thank you for the feedback as I am always looking for suggestions to be better at my training and racing. 

Thanks,

No worries. I had meant my comments to be more directed at the OP than necessarily at you in particular. We're each an experiment of one, and my perspective is definitely more from a road racing and long distance riding perspective, designing and building custom frames, and lastly from triathlon. As always, YMMV is the overarching credo.

That part should read: "you shouldn't immediately feel like you're falling forward." Apologies for any confusion.